This invention relates to novel derivatives of reducing sugars having molecular weights of up to 2000 which reducing sugars are C-methylolated in the .alpha.- and/or .alpha.'-position to the carbonyl group or to a carbonyl group which is masked by cyclohemiacetal formation. By reducing sugars are meant in the context of this invention carbohydrates which reduce Fehling's solution. Such carbohydrates include both natural sugars and derivatives of natural sugars. Specific families of useful carbohydrates include amino sugars, monosaccharides, disaccharides, trisaccharides and oligomeric polysaccharides obtained by partial acid or enzymatic hydrolysis of natural polysaccharides, e.g. of starch (i.e. amylose and amylopectin), cellulose, inulin, hemicelluloses, glycogens and those derived from wood (i.e. cellulose containing lignin). The invention also relates to a new and simple process for the preparation of these .alpha.-methylolated derivatives and to their use for various purposes in which their enhanced hydroxyl functionality is of particular interest.
It is known that various carbohydrates which carry cyclohemiacetal end groups and which reduce Fehling's solution (including monosaccharides such as grape sugar (i.e. .alpha.- and .beta.-glucose), fructose; disaccharides such as maltose, lactose, cellobiose, and the like; and tri- and oligosaccharides) are subject to rearrangement and decomposition reactions in the presence of alkalies or organic bases (see Langenbeck, Lehrbuch der Org. Chemie (1952), page 258 et seq). The nature and extent of these rearrangement, carmelization, dehydration and polymerization reactions, preliminary stages of carbonization reactions and Cannizzaro reactions between aldose and ketose sugars which take place in a basic medium, often accompanied by pronounced deepening of color, are generally unknown because the reaction and decomposition products are extremely difficult to isolate and are soluble only in water. Accordingly, the possibility of using spectroscopic methods which would identify such products is severely limited or may even be precluded.
Applicants have found that in aqueous solutions the carbohydrates noted above react with formaldehyde, generally at pH values above 7, to yield sugar hemiacetals corresponding to the following schematic structure, which vary from pentafunctional to octafunctional according to the quantity of formaldehyde present: ##STR1## wherein Z means the carbon skeleton of a sugar molecule and n means the number of methylolated hydroxy groups.
In aqueous solution, these sugar hemiacetals are in equilibrium with methylene glycol (HO--CH.sub.2 --OH). By removal of water at reduced pressure (e.g. from 0.06 to 15 Torr), the pure sugar polyhemiacetals can be obtained. These hemiacetals will, however, readily split off formaldehyde even by hydrolysis in a neutral medium or by heating. Cyclic and chain lengthening or chain branching or cross-linking acetal groups, which are stable to heat and to alkaline hydrolysis, can be introduced into the sugars by dehydration at pH below 5.5 which is accompanied by the elimination of water from the sugar molecules, but these acetal groups are readily split off by hydrolysis at acid pH.